Tile cache seems to forget tiles exist, momentarily

[jengelh@…]

Platform: josm 15937 (java 11.0.6 on openSUSE Leap 15.1 x86_64)

Reproduction 1:

• Load a background map. If the source is "slow" to load, the more easily the issue can be observed. (1-2 seconds per tile is a good start to trigger this, it need not be as slow as osm.org is tonight, though!) The initial zoom I start out with is z14.
• Zoom in a bit (went to z19 I think).
• Zoom out to z14.

Observed for reproduction 1:

• Screen capture of this happening: ​http://inai.de/files/zoomout.mkv [9.1MB].
• At 18s into the video, I zoom out again to z14, JOSM somehow forgets that it had z14 tiles in the cache - and loads them again. Not nice, but I could live with that.
• At 29s though, JOSM switches a large portion of the viewport to a much _coarser_ tile resolution — z10 as far as I can determine — for no apparent reason, even though some z14/z15 tiles are already available.
• In recurring tries, I found that, when zooming z14->z19->z13, JOSM would display z9 as the coarse level.

Reproduction 2:

• Stay at zoom level and just move the viewport.

Observation 2:

• Screen capture: ​http://inai.de/files/zoomout2.mkv [7.3M]

We start out at a location with a serpentine-ish stretch of road, then move westwards. Some tiles load, that's ok, they are new.

• At 12s, we go back eastwards and find that super-coarse z10 tiles are shown along the way (briefly, anyway), and
• (at 16s) the tiles for the bendy part are at z18 or something and z19 is re-requested.

Other info:

• Happens for TMS and WMS alike

[StephaneP]

Sometimes I see the "observation 2" problem too. Here is another video :

https://josm.openstreetmap.de/attachment/ticket/19004/zepnS2Eznn.mp4